In many machines, particularly machine-tools, there are one or several sliding members, adapted for moving by being guided by slides, on a support which can in turn slide, as is the case for example of a carriage sliding horizontally on a bracket which is in turn sliding vertically.
It is known how to reduce or cancel the action of gravity on the positioning device of such a member by a counterweight, a jack, a spring, etc.
The actions of the forces applied by said means on the unit to be balanced admit a resultant the point of application of which is generally fixed.
In the case of machines comprising a carriage supporting a further carriage, mobile on the first along any direction other than the vertical, the centre of gravity of the unit is displaced; the result is a variable distance between the point of application of the resultant of the balancing forces and the centre of gravity of the carriage unit which creates a couple of forces the moment of which causes the deformation of the guides, the uncontrolled displacement of the carriages within the play provided for their displacement in their slides, as well as antagonistic efforts in the positioning device.
To remedy such a problem the importance of which is increased due to the development of digital controlled machines, various solutions have already been proposed. In fact, on such machines, the positioning of the members is automatic, and it is therefore necessary to avoid or automatically correct the deformations, uncontrolled displacements or plays, which, previously, could be corrected by a manual adjustment.
According to one of such solutions disclosed in French Pat. No. 72.32413 (publication number 2.153.990), a counterweight is used, such that the position of its centre of gravity can be modified by shortening or increasing one of its suspension chains, which support said counterweight in distinct and distant points.
However, the modifications thus obtained are of small amplitude and can be used with good results only if the member which is mobile along a horizontal direction is of small weight, so that the displacement of the centre of gravity is of little importance.